To clarify...
A 30ml bottle of 30/70 mixture is 9ml of PG and 21ml of VG (based on ratio). To make 50/50, you need to raise the PG to equal the VG, thus adding 12ml to bring PG up to 21ml level (42ml total mixture). Furthermore, the Nicotine will be diluted by nearly 1/3 because you are increasing the volume nearly 50%... simple math tell you how much Nicotine will be affected... 6 * 30 / 42 = 4.28571mg
It shouldn't increase TH too much, but adding PG will maintain the flavor (for the most part)... PG is the flavor carrier, so increasing the volume of PG should maintain the flavor fairly well.